The thermal EMF spec is for the TXQ is 5uV and .3uV for the TX-S but those are for the non-latching parts. Are you using latching versions and if so have you measured the thermal EMFs (or tried - I guess it's quite tricky to do)?

I am not able to measure thermal EMFs reliable since my 24-Bit ADC is quite noisy. (1uVpp with a integration time of 1 minute).A partly compensation of the thermal EMFs is done in my setup since I am doing a offset measurement through the relays.I can actually switch the +/- signal of the output to every + pin of the references or to the common ground. So its possible to do difference measurements between 2 references also.

Some cleaning of the contacts is also done by daily switching of the relays.

finally received the Caddock MG745 40MOhm 0.1% resistors.In my first checks they are well within spec and spread positive and negative in value.That should help in combining them to a my specific values.

$3/ea, 5V, DPDT 2-coil bistable in through hole. According to the datasheet thermal emf is <10uV, which is as good as the baseline version of the Coto Low Thermal relays (much better I imagine though as they are latching so no internal heat generation).

1) Hayao Miyazaki - "there is no nuclear power here (at studio ghibli)" 2) Feynman - "What Do You Care What Other People Think?" 3) is it possible that the speed of light (a million years ago) is faster than it is now? or slower? 4) do not go to a WASET scientific conference!

The scanner of the Prema 5017 SC uses Omron G6AK-234P, just for the info. The scanner is specified with thermo forces typ. ±1µV, max. ±2 µV after 1,5 h warm-up. The Prema 2080 scanner, that I had, is specified the same way using the same relays.

Inspired by Zlymex, I started to design my own scanner based on the same idea.The goal is to make some improvements:

1. indicate active channel's number (1 of 16 LEDs). The measurements are made very slowly. If there is some bad connection etc. it will be easy to identify. I tested green LEDs that work satisfactory biased with 50uA of current.2. provide Manual control of the channel3. provide power-on reset (quite tricky part...)4. use RJ45 connectors, so the user just cuts off-the-shelf CAT6 cable in two pieces and strips wires only on one side. This also makes testing of relays easy - just disconnect RJ45 cables and plug in RJ45 plugs with pairs shorted by loop of wire. The RJ45 connectors will be hidden inside metal enclosure to ensure thermal equilibrium.5. use SMD components (prepared to reflow soldering, better EMF performance expected)6. the board should fit standard TUFxx Fischer Elektronik enclosure7. It's nice to have a spare relay on board (tribute to Data Proof scaners)8. the design should be expandable to 2 wire 32 channels; 4 wire 16 channels; 4 wire 32 channels by using the same boards (with some PCB jumpers, desoldering of some components but basically same design)9. last but not least: the scanner shall provide break-before-make action not to disturb expensive gear and save the relays.

The quiz is: Open the schematic pdf file and find as many bugs as possible.If anyone is interested in factory made PCB for this project let me know ->PM. Zlymex has a free PCB if he wants

First bug found: net NE0 added to J5 connector. Without it the slave board would not break-before-make. Schematic upgraded to rev 0.2

Edit:SW1 pin 6 connected to wrong node.Schematic upgraded to rev 0.3

Edit: Switched from 74HC4538 to 74HC123 for two reasons:4538 has built-in power-on reset circuit which would disable generation of a pulse after turning scanner on. This is not wanted in this circuit.4538 draws much more idle current.Schematic upgraded to rev 0.4

You can't compare any Vref against any other Vref: they are separated into two groups, and any Vref from group "A" can be compared to one from group "B".

With this restriction, you can get away with only two DPDT relays per Vref, which is pretty cheap.

Here, four Vrefs are shown, but the idea is that any number of Vrefs is supported.

The DMM also uses two SPDT relays and can do the following:

Measure CH A to CH GND (i.e. measure a single Vref from group "A")

Measure CH B to CH GND (i.e. measure a single Vref from group "B")

Measure CH A to CH B (i.e. measure the difference between a Vref from group "A" and group "B")

Just to walk through some examples:

To measure Vref 1:

Vref 1 input is switched from SUPPLY to BATTERY A

Vref 1 output is enabled, placing it across CH A - CH GND

DMM inputs are switched to CH A and CH GND

To measure Vref 2:

Vref 2 input is switched from SUPPLY to BATTERY B

Vref 2 output is enabled, placing it across CH B - CH GND

DMM inputs are switched to CH B and CH GND

To measure Vref 1 against Vref 2:

Vref 1 input is switched from SUPPLY to BATTERY A

Vref 2 input is switched from SUPPLY to BATTERY B

Vref 1 output is enabled, placing it across CH A - CH GND

Vref 2 output is enabled, placing it across CH B - CH GND

DMM inputs are switched to CH A and CH B

Edit: the caps on each Vref are large enough that they can be switched from SUPPLY to BATTERY A/B without losing power. This is a somewhat simplified schematic, as there would probably a regulator after each capacitor, keeping a stable voltage supplied to each Vref.

Edit 2: to elaborate, Vrefs 1, 3, ... are in group "A" and Vrefs 2, 4, ... are in group "B". So you can measure Vref 1 against 2 or 4, but not against 3. To get the difference between 1 and 3, you'd have to do it indirectly by measuring 1-2 and 3-2, then calculate 2-3. With a large number of Vrefs, many cross-checks are possible, so errors in the system should become apparent.

Edit 3: the batteries can be charged, but all Vrefs must be on SUPPLY, so no (floating) measurements can take place while the batteries are being charged.

Dont tell me that scanner-mainframe you just bought with the load of 20-relay-cards dont suffice cellularmitosis

The scanner gets me started immediately with something that works, but does no good for the community! Ideally I can produce some smaller scanner kits, benchmark them against the Keithley cards with the COTO relays, and publish an open source design to help out fellow volt nuts

Buying old gear is nice, but reproducing equivalent results in a DIY project is a lot more satisfying

Hmm, since my old Keithley 7064 card produces unstable readings in some configurations i would be interested in some new cards. The easiest solution for me would be designing keithley 705-compatible cards (defined lengths and a 15pin-card-edge-connector), but then i have the problem that they were build to drive non-latching relays and i would have to add drive logic on the cards to use latching-relays. Meh.

Dont tell me that scanner-mainframe you just bought with the load of 20-relay-cards dont suffice cellularmitosis

The scanner gets me started immediately with something that works, but does no good for the community! Ideally I can produce some smaller scanner kits, benchmark them against the Keithley cards with the COTO relays, and publish an open source design to help out fellow volt nuts

Buying old gear is nice, but reproducing equivalent results in a DIY project is a lot more satisfying